In recent years, the treatment of agricultural wastewater has been an important aspect of environmental protection. The purpose of photocatalytic technology is to degrade pollutants by utilizing solar light energy to stimulate the migration of photocarriers to the surface of photocatalysts and occur reduction-oxidation reaction with pollutants in agricultural wastewater. Photocatalytic technology has the characteristics of high efficiency, sustainability, low-energy and free secondary pollution. It is an environmental and economical method to recover water quality that only needs sunlight. In this paper, the mechanism and research progress of photocatalytic removal of heavy metal ions and antibiotics from agricultural water pollution were reviewed by combining photocatalytic degradation process with agricultural treatment technology. The mechanism of influencing factors of photocatalytic degradation efficiency was discussed in detail and corresponding strategies were proposed, which has certain reference value for the development of photocatalytic degradation.

In this work, an all-fiberized and narrow-linewidth fiber amplifier with record output power and near-diffraction-limited beam quality is presented. Up to 6.12 kW fiber laser with the conversion efficiency of approximately 78.8% is achieved through the fiber amplifier based on a conventional step-index active fiber. At the maximum output power, the 3 dB spectral linewidth is approximately 0.86 nm and the beam quality factor is M_x² = 1.43, M_y² = 1.36. We have also measured and compared the output properties of the fiber amplifier employing different pumping schemes. Notably, the practical power limit of the fiber amplifier could be estimated through the maximum output powers of the fiber amplifier employing unidirectional pumping schemes. Overall, this work could provide a good reference for the optimal design and potential exploration of high-power narrow-linewidth fiber laser systems.

In this work, we analyze the effects of the background spectral noise in phase-modulated single-frequency seed lasers on the spectral purity of high-power narrow-linewidth fiber amplifiers. Through demonstrating the spectral evolution of the phase-modulated single-frequency part and the background spectral noise in a narrow-linewidth fiber amplifier, the mechanism for the spectral wing broadening effect is clarified and design strategies to maintain high spectral purity are given. Specifically, the background spectral noise in phase-modulated single-frequency seed lasers could lead to obvious spectral wing broadening and degeneration of spectral purity in narrow-linewidth fiber amplifiers through the four-wave-mixing effect. Notably, the spectral wing broadening effect could be suppressed by filtering out the background spectral noise in the seed laser or applying a counter-pumped configuration in the fiber amplifier. We have also conducted contrast experiments, which have verified the validity of the theoretical model and the design strategies for high-spectral-purity operation.

Quantum defects (QDs) have always been a key factor of the thermal effect in high-power fiber lasers. Much research on low-QD fiber lasers has been reported in the past decades, but most of it is based on active fibers. Besides, Raman fiber lasers based on the stimulated Raman scattering effect in passive fiber are also becoming an important kind of high-power fiber laser for their unique advantages, such as their significantly broader wavelength-tuning range and being free of photon darkening. In this paper, we demonstrate an ultralow-QD Raman fiber laser based on phosphosilicate fiber. There is a strong boson peak located at a frequency shift of 3.65 THz in the Raman gain spectrum of the phosphosilicate fiber we employed. By utilizing this boson peak to provide Raman gain and adopting an amplified spontaneous emission source at 1066 nm as the pump source, 1080 nm Stokes light is generated, corresponding to a QD of 1.3%. The spectral purity at 1080 nm can be up to 96.03%, and the output power is 12.5 W, corresponding to a conversion efficiency of 67.2%. Moreover, by increasing the pump wavelength to 1072 nm, the QD is reduced to 0.74%, and the output power at 1080 nm is 10.7 W, with a spectral purity of 82.82%. To the best of our knowledge, this is the lowest QD ever reported for Raman fiber lasers. This work proposes a promising way of achieving high-power, high-efficiency Raman fiber lasers.